(a) Field of the Invention
This invention relates generally to the art of amperometric or polarographic measurement and amperometric or polarographic measuring devices of the type used for quantitative electrochemical analysis methods where the concentration of an electroactive species such as oxygen dissolved in or admixed with a fluid such as a gaseous or liquid medium is to be measured or monitored; more particularly, this invention relates to membrane-enclosed amperometric or polarographic cells and to methods of mounting and securing the membrane constituent on such cells.
(b) Description of the Prior Art
Electrochemical cells or transducers of the type used for quantitative electrochemical analysis are well known in the art and generally include a working or sensing electrode having a defined or electroanalytically effective surface, a counter electrode, an electrolyte in contact with the electrodes, a barrier means that is substantially impervious to the electrolyte but is permeable to a gas (i.e. a "semi-permeable membrane") and a cell structure or housing for receiving and holding the cell constituents in operative connection.
For amperometric analytical operation, the working electrode in a transducer-type cell arrangement is polarized by a constant DC voltage to furnish a current whose steady state magnitude is proportional to the activity of the chemical substance of interest. Transducers of this type and their operation and uses are discussed in detail in the following illustrative U.S. Pat. Nos. 2,913,386, 3,071,530, 3,223,608, 3,227,643, 3,372,103, 3,406,109, 3,429,796, 3,515,658 and 3,622,488.
The first mentioned U.S. Pat. No. 2,913,386 to Leland E. Clarke considered as the pioneering patent in this art already teaches a barrier means in the form of a semi-permeable membrane formed of a flexible polymer such as polyethylene and the terms "membrane-covered" or "membrane-enclosed" are being used generally to refer to such electroanalytical devices, e.g. as "membrane-covered polarographic detectors."
As the term "polarography" has also been used for techniques based on the dropping mercury electrode and operating either in a voltametric or galvanic mode, the term "membrane-enclosed amperometric cell" or MEAC is used herein to refer to electroanalytical devices of interest here such as the "Clark Cell" and modifications thereof.
A structural feature common to most prior art MEAC's is a generally cylindrical outer structure of the cell, e.g. provided by an elongated tubular cell housing or jacket that receives and holds the above mentioned cell components; as the electroanalytically effective surface of the sensing electrode is the site where the measuring signal is generated, and as this surface must be covered by an electrolyte layer formed between the membrane and the surface of the sensing electrode, the sensor face of a MEAC will generally be that cell portion where the electroactive species of interest can get through the membrane and the electrolyte layer on top of the sensing electrode surface to that electrode surface.
For convenience of construction as well as for structural and operative reasons, the sensor face of a MEAC generally is a front side of the cell structure, that is, generally vertically intersecting with the cell axis. As will be explained below in more detail, the frontal or "transversely" extending sensor face of a MEAC can be plane or curved ("calotte-shaped") and both types are known in the art.
As is also known in the art, a convenient method of securing a polymer membrane on the electrolyte-bearing sensor face of a MEAC is to use an annular holding member that presses a peripheral portion of a substantially circular polymer film onto the cylindrical end of the MEAC adjacent or near the sensor face.
For example, Clark in U.S. Pat. No. 2,913,386 discloses a cap-type holding member that presses an O-ring seal against the membrane and the cylindrical outer wall of the cell housing. Because one of the most frequent operations in the maintenance of a MEAC is exchange of the electrolyte and as this involves removal and replacing of the membrane, several modifications of the membrane-mounting and holding structure and method disclosed by Clark have been suggested and tried, cf. the elastic tubular holding member or the elastic O-ring sealingly retaining the stretched membrane in a cylindrical outer cell wall portion as disclosed by D. A. Okun et al. in U.S. Pat. No. 3,227,643, the disc-clamping device disclosed by J. A. Porter et al. in U.S. Pat. No. 3,445,369 and the membrane-holding cap assembly disclosed by T. M. Doniguian in U.S. Pat. No. 3,758,398.
Doniguian, after illustrating and explaining the disadvantages of prior art membrane mountings by means of an O-ring seal suggests incorporation of the membrane into a pre-assembled tubular cap for threading engagement with the cell housing. The periphery of the membrane is clamped and sealingly held between an internal bore of the cap and a tubular holding ring tightly fitting into the cap bore. For membrane mounting, the membrane/cap-assembly is supplied with electrolyte; then, the assembly is threaded onto the cell so that the calotte-shaped sensor face will forcibly stretch the membrane while retaining a thin film of electrolyte between the membrane and the sensing electrode.
An important further factor must be considered, however, when reviewing the problems of mounting a given semi-permeable membrane on a MEAC, i.e. the thickness or gauge of the membrane and its effect upon operation of a MEAC. For example, according to U.S. Pat. No. 3,227,643 a typical thickness of a polyethylene membrane is 1 mil (25 micrometers); a typical preferred thickness in the range of from 10 to 20 micrometers has been disclosed in our above mentioned U.S. Pat. No. 4,096,047 for high-tenacity polymers, such as polytetrafluoro ethylene, and membrane thicknesses of as low as 0.2 to 2 micrometers have been discussed in the literature (cf. M. L. Hitchman, Measurement of Dissolved Oxygen, ISBN 0471 03885-7; incorporated herein by way of reference); the main advantage of using thinner membranes is that the response time to step changes in the concentration of the measured species is decreased. An added advantage of relatively thin (i.e. up to 25 micrometers) membranes is that they can be mounted and secured on the cell by means of simple O-rings without substantial problems caused by wrinkling. On the other hand, the sensitivity of such thin membranes is substantial. For example, simply touching the normal membrane of a MEAC by hand can cause a change in membrane stress that can necessitate recalibration. Further, any comparatively rough treatment, such as brushing away an algae layer from such a thin membrane, may cause irreparable membrane damage.
When attempting to use relatively thicker membranes--for example when minimum response time to step change is less important than membrane stability--prior art membrane-mounting methods are not suitable; either--e.g. in the case of conventional O-ring seals--the problem of wrinkling or folding of the membrane at or near the sensor face and/or lack of sealing cannot be resolved or--e.g. with membrane/cap-assemblies--the stress of the membrane may cause time-dependent permeability changes aside from the relatively complicated and bulky structure of prior art membrane assemblies.